Wholly aromatic polyamide foam

ABSTRACT

Aromatic polyamide foams having a density of 0.025 to 0.7 g./cm.3 are prepared by heating N-alkyl aromatic polyamides having at least one beta  hydrogen atom on the alkyl groups, a plasticizer and an aromatic sulfonic acid catalyst at 250 DEG  to 340 DEG  C. for a length of time sufficient to remove substantially all of the N-alkyl groups.

RELATED APPLICATION

This application is a continuation-in part of application Ser. No.881,731 filed Feb. 27, 1978, now U.S. Pat. No. 4,178419 issued Dec. 11,1979.

BACKGROUND OF THE INVENTION

This invention relates to rigid aromatic polyamide (aramid) foams and aprocess for their preparation.

Aromatic polyamides are known to have many desirable properties such asgood resistance to oxidation at high temperature, a high melting point,low flammability, high stiffness and low sensitivity to solvents. Foammaterials are known to be useful as thermal insulators and aramid foamssuggest themselves for this use but the high melting points and lowsolubility of the aromatic polyamides makes fabrication of such foamsdifficult.

U.S. Pat. No. 3,770,669 discloses one approach to the preparation ofpolyamide foams. According to this patent polyamides are prepared fromN,N'-dialkyl diamines and dicarboxylic acids or dicarbonyl chlorides.The resulting N,N'-dialkyl polyamides are heated at temperatures up to380° C., optionally in the presence of a salt of a strong acid and aweak base, e.g., pyridinium hydrochloride, to give a foam product. Anexample of an aramid foam is included. However, it has been found thatthis product is brittle and excessively flammable, apparently because ofan insufficiently high molecular weight and incomplete dealkylation,respectively.

Foam materials combining the features of good mechanical strength andresistance to degradation at high temperatures in the presence of airhave long been sought after. Such foams are particularly needed in useswhere high temperatures are encountered in structures wherein mechanicalstrength of the foam is required. One such use is in solar collectorsfor the utilization of solar energy.

BRIEF SUMMARY OF THE INVENTION

This invention provides foam materials having a density of from 0.025 to0.7 g./cm.³ which combine the features of good mechanical strength, lowflammability and resistance to degradation in air at high temperatures.

This invention provides an aromatic polyamide foam having a density of0.025 to 0.7 g./cm.³ wherein the cell walls are oriented as evidenced bya birefringence of at least 10% of the maximum possible birefringence.This invention also provides an aromatic polyamide foam prepared bydealkylation of an N-alkyl aromatic polyamide having a density of 0.025to 0.7 g./cm.³ and having substantially no residual N-alkyl groups.Preferably either the diamine residue or the dicarboxylic acid residueis para-oriented, most preferably both the diamine residue and thedicarboxylic acid residue are paraoriented and still most preferably thearomatic polyamide is poly(p-phenylene terephthalamide) and the walls ofthe cells comprising the foam have texture.

The foams of this invention are prepared by heating a mixture of anN-alkyl aromatic polyamide having at least one β hydrogen atom on thealkyl group and a plasticizer at a temperature of 250 to 340° C. for atime sufficient to remove substantially all of the N-alkyl groupswherein the mixture contains an aromatic sulfonic acid as catalyst.Preferably the heating is for 150 to 300 minutes. Preferably,substantially all the N-alkyl groups are unsymmetrical and contain 4 to8 carbon atoms. Preferably the plasticizer is o-dichlorobenzene and thecatalyst is benzenesulfonic acid. Most preferably, the alkyl groups aresec-butyl. Most preferably the N-alkyl aromatic polyamide ispoly(N,N'-di-sec-butyl-p-phenylene terephthalamide).

DETAILED DESCRIPTION OF THE INVENTION

The N-alkyl groups of the aromatic diamines suitable for preparing thestarting polyamide should have 2 to 8 carbon atoms and at least oneβ-hydrogen atom. Alkyl groups having at least 4 carbon atoms and anunsymmetrical structure are preferred since these diamines provideN-alkyl aromatic polyamides having lower melting points and greatersolubility than alkyl groups having fewer carbon atoms and/or asymmetrical structure. N,N'-dialkylphenylene-diamines are preferred andN,N'-dialkyl-p-phenylene-diamines are most highly preferred. SuitableN-alkyl groups are ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl,t-butyl, pentyl, octyl, 1-ethyl-3-methyl-pentyl, 1-methyl-heptyl,cyclohexyl, 1,4-dimethylpentyl, and 1,3-dimethylbutyl. Preferreddicarbonyl chlorides are isophthaloylchloride, terephthaloyl chloride,2,6-naphthalene dicarbonyl chloride, 4,4'-bibenzoyl chloride,3,4'-bibenzoyl chloride and substituted derivatives thereof wherein thesubstituents are inert to the polymerization and foam forming reactions.

The N-alkylated aromatic polyamide starting materials are best preparedby reaction of one or more of the above mentioned N-alkylated aromaticdiamines with an aromatic dicarbonyl chloride at high temperature (about150° C.) in a solvent such as mineral oil, o-dichlorobenzene,1,2,4-trichlorobenzene, diphenylether, or pyridine. Optionally, atertiary amine acid acceptor may be used. The N-alkylaromatic polyamidesare conveniently isolated by cooling the mixture whereupon the polymerprecipitates or by precipitation with a nonsolvent such as hexane. TheN-alkyl aromatic polyamides should have an inherent viscosity of atleast 0.25. The amount of N-substitution should be sufficient to providea polymer having a suitable melting point.

The preparation of the foam materials is conveniently carried outstarting from a solution or suspension of the N-alkylaromatic polyamidein a suitable solvent such as o-dichlorobenzene. Other suitable solventsare 1,2,4-trichlorobenzene and diphenyl ether. If these solvents areused for the polymerization reaction the polymer need not be isolated.

The catalyst for the dealkylation is an aryl sulfonic acid present in anamount of about 1 to 10% wt. based on the N-alkyl aromatic polyamide.Suitable aryl sulfonic acids are e.g., benzenesulfonic acid,p-toluenesulfonic acid, o-toluenesulfonic acid, m-toluene sulfonic acid,xylene-sulfonic acids, α or β napthalene sulfonic acid etc.Benzenesulfonic acid is preferred.

The mixture of N-alkyl aromatic polyamide, solvent and catalyst isheated while permitting the solvent to distill and then further heatedat a temperature of 250° to 340° C. to effect dealkylation and createthe foam. In order to avoid degradation of the polymer, the heatingshould be for as short a time and at as low a temperature as arecommensurate with obtaining a substantially dealkylated product.Incompletely dealkylated products are excessively flammable, apparentlybecause of further alkene being split off under conditions ofcombustion.

It is ordinarily desirable to complete the dealkylation reaction underreduced pressure which also assists in foam formation.

The foams of the present invention have good mechanical strength havinga compressive strength of at least 100 lb./in.² (690 kPa).

The cells of the foam are closed and range in size from a few microns toa few mm. in diameter.

Optionally the foams may contain up to 30% by weight of inert fillers,antioxidants, UV screeners etc. Preferably any fillers used are of lowdensity, so as not to increase the density of the foam.

Tests Inherent Viscosity

Inherent viscosity (ηinh) is defined by the following equation:

    ηinh=(1n (ηrel)/C)

wherein (ηrel) represents the relative viscosity and C represents aconcentration of 0.5 grams of the polymer in 100 ml. of solvent. Therelative viscosity (ηrel) is determined by dividing the flow time in acapillary viscometer of a dilute solution of the polymer by the flowtime for the pure solvent. The dilute solutions used herein fordetermining (ηrel) are of the concentration expressed by (C), above;flow times are determined at 30° C.; the solvent is concentratedsulfuric acid (96-98% H₂ SO₄).

Limiting Oxygen Index (LOI)

The L.O.I. is the minimum fraction of oxygen in an oxygen/nitrogenmixture required to just support burning of the sample, see Fenimore andMartin, Modern Plastics, 44 (3), 141 (1966).

Mechanical Strength

Compressional strength and modulus are determined by ASTM D-1621-73.

Density

Density is determined by ASTM D-1622.

Melting Point

A melting point of greater than 400° C. indicates substantially completedealkylation. The melting point can be determined on a hot bar having atemperature gradient. The melting point is taken as the temperature atwhich the sample sticks to the bar.

Residual N-alkyl Groups

It is frequently possible to follow the dealkylation reaction byinfrared spectroscopy noting loss of absorption at 3.35μ (2960 cm⁻¹).When substantially all of the N-alkyl groups have been removed there issubstantially no absorption peak at this frequency and the dealkylatedpolymer has a melting point and LOI at least as high as that of theunderlying polymer.

Orientation in Cell Walls

In this test it is assumed that one knows the birefringence of a highlyoriented fiber made of the polymer that comprises the foam beinginvestigated. Cell walls of the foams of this invention have eitheraxial orientation (molecules aligned in one direction) in which thebirefringence in the plane of the wall is at least 10% of maximum, orplanar orientation (all molecules parallel to a plane) in which thebirefringence perpendicular to the plane of the wall is at least 10% ofmaximum. The test may be performed by the following or equivalent steps.

(1) Remove a cell wall from the sample using a razor blade and tweezers.Place it on a microscope slide

(2) Using an interference microscope, find the wall thickness at thepoint to be investigated.

[Hale, A. J. The Interference Microsope in Biological Research E & SLivingstone. Ltd. Edinburgh, 1958. (Page 77)]

(3) Using the wall thickness from (2) find the birefringence in theplane of the wall using a 1/4 wave plate or other compensator

[Hallimond, A. F. The Polarizing Microscope (3rd edition) Vickers Ltd.York 1970. (Page 70)]

(4) If the birefringence in (3) is less than 10% of maximum find thebirefringence perpendicular to the plane of the wall by using auniversal stage and tilting techniques.

[Same reference as (3) (Page 187, see also Hartshorm, N. H. and Stuart,A. Crystals and the Polarizing Microscope, Edw. Arnold Ltd., London 1960p. 406.]

Texture

When the cell wall is observed between crossed polars in a microscope,if no position can be found where the wall extinguishes (goes black)uniformly, the cell wall has texture.

EXAMPLE 1

To 75 ml of o-dichlorobenzene is added 22.0 g (0.1 mol)N,N'-di-sec-butyl-p-phenylene diamine (N,N'-D-s-BuPPD) and 20.3 g (0.1mol) terephthaloyl chloride (TCl). The clear raction mixture is heatedat reflux under N₂ purge for 8 days. The mixture containingpoly(N,N'-di-sec-butyl-p-phenylene terephthalamide) (N,N'-D-S-BuPPD-T)is allowed to cool to the point where it can be poured without boiling.Half of the mixture is transferred to a polymer tube to which an N₂capillary and receiver with vacuum line are connected. Benzene sulfonicacid (technical grade 90%), 0.44 g (2.4% by weight) is added and isheated under N₂ in vapor baths at the following temperatures andpressures for the times shown: 220° C./N₂ purge/5.3 h; 259° C./N₂purge/1.5 h; 259° C./380 mm Hg (51 KPa)/17 h. During the heating thesolvent is mostly distilled at 222° C. and the foam began to form at259° C. The tube is allowed to cool to room temperature and is broken toremove the foam. The yield is 12 g. This corresponds to quantitativeconversion of the N,N'D-s-BuPPD-T to poly(p-phenylene terephthalamide)(PPD-T) based on using about half of the initial reaction mixture in thefoaming stage. The infrared spectrum of the foam is identical to that ofPPD-T from p-phenylene diamine and terephthaloyl chloride. The densityof the foam is about 0.1 g/cm³. The limiting oxygen index is 0.36.Again, this is consistent with the PPD-T structure. A similar foam wasprepared at atmospheric pressure.

EXAMPLE 2 Preparation of N,N'-D-s-BuPPD-T

N,N'-D-s-BuPPD is distilled twice through an 18 in-1 in inside diameterpunched column. The boiling point is 123° C. at 0.6 mm. Hg. TCl wasrecrystallized from hexane.

Procedure

To 1800 ml o-dichlorobenzene in 3 liter 3 necked round bottom flask isadded 440 g (2 mol) N,N'-D-S-BuPPD (98.8% pure by gas chromatography)and 406 g (2 mol) TCl. The reaction is heated to reflux during whichtime a clear yellow solution is formed at 100° C. The reaction is heatedunder reflux for 12 days then cooled to room temperature. The solidifiedmass is washed 3 times with hexane in a Waring blender using a total of3.5 gal of hexane. The polyamide is removed by filtration on a sinteredglass Buchner funnel. The filter cake is dried in a 90° C. vacuum oven.Yield-725 g (103%)* η_(inh) -0.40 (H₂ SO₄).

Foam Preparation

To a 2 liter resin kettle fitted with a Claisen head and mechanicalstirrer (glass shaft with Teflon® fluorocarbon stirrer blade) is added500 ml o-dichlorobenzene, 250 g N,N'-D-s-BuPPD-T, ηinh (H₂ SO₄)--0.40and 5.4 g benzenesulfonic acid (2.1% by weight) (90% tech. grade -Aldrich). The kettle is heated with stirring in a Woods metal bath at200° C. under N₂ for 20 minutes when a homogeneous solution is obtained.The solution is heated for 20 minutes more at 200° C. and then thetemperature is raised to 220° C. and the o-dichlorobenzene begins todistill. After 150 minutes at 220° C., the temperature is raised to 255°C. and held there for 35 minutes. The viscous yellow melt containing 60ml. of the initial solvent is poured into a 2.5 cm deep 20×20 cm square"Pyrex" dish. The melt solidifies immediately. The dish containing thesolid is placed in a Berringer vacuum oven and is heated as followsunder 275 mm Hg. vacuum: 20 minutes/270° C.; 110 minutes/310° C.; and 30minutes/337° C. The vacuum is released and the dish containing the foamremoved. The foam has average dimensions of 20×20×2.5 cm a weight of 150g and a density of 0.15 g/cm³ (9.35 lb/ft³).

A similar foam is prepared in similar fashion except only 25 ml ofo-dichlorobenzene was left in melt. The foam had the followingproperties.

    ______________________________________                                         Compressive Strength                                                         (10% compression)  1.0 MPa (150 lbs/in.sup.2)                                 Compressive Modulus                                                                              32 MPa (4700 lbs/in.sup.2)                                 Limiting Oxygen Index                                                                            0.30                                                       Density            0.16 g/cm.sup.3                                                               (10 lb/ft.sup.3)                                           ______________________________________                                         birefringence, 0.04, 0.04, 0.06,  10%, 10%, 15% max. The cell walls have      texture.                                                                 

EXAMPLE 3 Poly(N,N'-di-sec-butyl-p-phenylene isophthalamide)(N,N'-D-s-BuPPD-I) Preparation

To 1800 ml o-dichlorobenzene is added 440 g (2 mol) N,N'-D-s-BuPPD and406 g (2 mol) isophthaloyl chloride (ICl). The mixture is heated toreflux and the clear solution held under reflux for 7 days. Uponcooling, the mixture remained a clear yellow solution. A small amount ofpolymer was isolated by coagulation with hexane. η_(inh) (H₂ SO₄)=0.3

Foam Preparation

To 1085 g of the above solution is added 5.4 g benzenesulfonic acid (90%technical grade) and the solution transferred to a 2 liter 3 neckedround bottom flask equipped with an N₂ bleed, a mechanical stirrer("Teflon" stir blade) and a Claisen head with a condenser and a DeanStark trap. The mixture is heated with an electric heating mantle toreflux and 550 ml. of o-dichlorobenzene is distilled off. The viscousmelt is poured into a 20×20×5 cm pyrex dish where it solidifies. Thedish is transferred to a Berringer oven and heated as follows:

    ______________________________________                                                     Oven                                                             Time         Temp. °C.                                                                          Pressure (N.sub.2 atm)                               ______________________________________                                         25 min      210         275 mm (37 KPa)                                      115 min      310         "                                                     30 min      337         "                                                    ______________________________________                                    

Yield--150 g of foam. The infrared spectrum shows only a trace ofN-alkyl groups remaining. The limiting oxygen index is 0.31. Thecompressive strength is 1.7 MPa (240 lbs./in.²) and the compressivemodulus 34 MPa (4960 lbs./in.²).

EXAMPLE 4 Preparation of poly(m-phenylene isophthalamide) (MPD-I)Containing About 32% N-Isobutyl Groups

To a 1 liter resin kettle with a shear disc stirrer and N₂ bleed isadded 200 ml of N,N-dimethylformamide (DMF), 20.5 g (0.20 mol) triethylamine and 14.3 g (0.1 mol) of m-phenylene diamine containing N-isobutylgroups on 32% of the nitrogen atoms (determinted by NMR). The mixture iscooled with stirring to 0° C. in an ice bath. To the clear solution isadded 20.3 g(0.10 mol) of isophthaloyl chloride. The mixture becomescloudy immediately. The mixture is stirred at 0° C. for 5.5 hours andthen allowed to warm to room tempearture during the next hour. Water(275 ml) is added and a curdy precipitate forms. The polymer is removedby filtration. The filter cake is washed with water, air dried on thefilter and then dried at 90° C. in a vacuum oven. Yield 26.5 g (97% oftheoretical). The η_(inh) is 0.26 in concentrated H₂ So₄. The polymermelts on a hot bar at 240° C.

Preparation of MPD-I Foam

To a polymer tube equipped with an N₂ capillary, a receiver and a vacuumline is added 2.5 g of the above poly(m-phenylene isophthamide)containing N-isobutyl groups, on 32% of the amide nitrogen atoms, 0.1 gof benzenesulfonic acid (90% technical grade) and 14 mlo-dichlorobenzene. The polymer tube with N₂ flowing through capillary isthen heated in a 222° C. vapor bath for 105 minutes during which timevigorous distillation of the o-dichlorobenzene occurs. The melt is thenheated in a 283° C. vapor bath for 145 minutes at a pressure of 380 mmHg (51 KPa). At this point 0.1 g further benzene sulfonic acid is addedand the system heated in a 305° C. vapor bath for 20 minutes at apressure of 380 mm Hg (51 KPa). The system is heated still further for120 minutes at 305° C. at a pressure of less than 1 mm Hg (133 Pa).During heating under these conditions the material foams and sets up toa solid. Upon cooling under N₂ and breaking the polymer tube 1.6 g of atough foam is obtained. The thin cell walls near the top of the sampleare flexible. The foam does not melt at temperatures up to 400° C. (hotbar) and is self extinguishing.

An attempt was made to prepare a similar foam as above except by heatingat 380° C. for 30 minutes without added benzenesulfonic acid catalyst. Adark brown melt giving off orange vapors was formed. The product was abrittle cellular material with an oily substance on the surface. Thefoam melted at 270° C. and had appreciable N-alkyl groups remaining asindicated by infrared spectroscopy. The foam could be ignited in air butwas self extinguishing.

I claim:
 1. A wholly aromatic polyamide foam prepared by dealkylation ofan N-alkyl wholly aromatic polyamide wherein either the diamine residueor the dicarboxylic acid residue is para oriented, said foam having adensity of 0.025 to 0.7 g./cm.³ wherein the cell walls are oriented asevidenced by a birefringence of at least 10% of the maximum possiblebirefringence and having substantially no residual N-alkyl groups. 2.The foam of claim 1 wherein both the diamine residue and thedicarboxylic acid residue are para-oriented.
 3. The foam of claim 2wherein the aromatic polyamide is poly(p-phenylene terephthalamide). 4.The foam of claim 3 wherein the walls of the cells comprising the foamhave texture.